7.1 Example of a Conditional

The following example of a conditional tells make to use one
set of libraries if the CC variable is ‘gcc’, and a
different set of libraries otherwise. It works by controlling which
of two recipe lines will be used for the rule. The result is that
‘CC=gcc’ as an argument to make changes not only which
compiler is used but also which libraries are linked.

This conditional uses three directives: one ifeq, one else
and one endif.

The ifeq directive begins the conditional, and specifies the
condition. It contains two arguments, separated by a comma and surrounded
by parentheses. Variable substitution is performed on both arguments and
then they are compared. The lines of the makefile following the
ifeq are obeyed if the two arguments match; otherwise they are
ignored.

The else directive causes the following lines to be obeyed if the
previous conditional failed. In the example above, this means that the
second alternative linking command is used whenever the first alternative
is not used. It is optional to have an else in a conditional.

The endif directive ends the conditional. Every conditional must
end with an endif. Unconditional makefile text follows.

As this example illustrates, conditionals work at the textual level:
the lines of the conditional are treated as part of the makefile, or
ignored, according to the condition. This is why the larger syntactic
units of the makefile, such as rules, may cross the beginning or the
end of the conditional.

When the variable CC has the value ‘gcc’, the above example has
this effect:

foo: $(objects)
$(CC) -o foo $(objects) $(libs_for_gcc)

When the variable CC has any other value, the effect is this:

foo: $(objects)
$(CC) -o foo $(objects) $(normal_libs)

Equivalent results can be obtained in another way by conditionalizing a
variable assignment and then using the variable unconditionally: